Method for manufacturing a double-walled heat exchanging tube with leak detection

ABSTRACT

Method for manufacturing a double-walled heat exchange tube with leak detection, wherein inner tube is slipped into an outer tube, after a surface profiling has been provided on at least the outer surface of the inner tube or the inner surface of the outer tube, and at least the outer surface of the inner tube or the inner surface of the outer tube has been provided with a layer of soldering material. After the tubes have been slipped into one another, the inner tube is expanded such that the outer tube is expanded as well and the surface profiling forms a leak detection channel between the two tubes, and the soldering material between inner and outer tube is caused to melt. In a heat exchange tube thus obtained at the location of the contact between the inner and outer tube, a film-thin, optionally porous layer from soldering material is present, which layer is bonded, through melting, to both the inner tube and the outer tube.

This invention relates to a method for manufacturing a double-walledheat exchange tube with leak detection, wherein an inner tube is slippedinto an outer tube, after a surface profiling has been provided on atleast the outer surface of the inner tube or the inner surface of theouter tube, and after the inner and outer tubes have been slipped oneinto the other, the inner tube is expanded such that the outer surfaceof the inner tube is in intimate contact with the inner surface of theouter tube and the surface profiling forms at least one leak detectionchannel between the two tubes.

Such a method is known from DE-A-30 00 665. In that method, a surfaceprofiling is provided on the outer surface of the inner tube in the formof a serration with a great multiplicity of pointed, pyramid-shaped orcone-shaped projections. To obtain a proper heat transfer, uponexpansion of the inner tube slipped into the outer tube, the tips of thevarious projections are pressed into the inner wall of the outer tube.Although as a result of such impression the magnitude of the contactsurface between inner and outer tube is in the order of an unworkedcontact surface, the resulting heat transfer, in comparison with aone-piece heat exchange tube without leak detection, can be qualified asdisappointing, while that heat transfer moreover deteriorates accordingas the heat exchange tube is longer in use.

To obtain an improved heat transfer, it is therefore proposed inDE-C-3706408 to fill up the leak detection channel with aheat-transferring fluid. As appears from the test diagram, although theheat transfer is thereby improved, it still remains considerably belowthat of a one-piece heat exchange tube without leak detection. Inaddition, this known composite heat exchange tube must meet particularconditions to maintain the leak detection function. The leak detectionchannel should be designed as a capillary gap and the heat-transferringfluid should have a boiling point above the maximum operatingtemperature of the heat exchange tube. Only then, due to the capillaryaction, will the fluid normally not run from the leak detection channel,but be forced therefrom in the event of leakage and thus indicate thepresence of a leak. Not only is this a complicated system imposingspecific requirements, but, moreover, it remains to be seen whetherexpansion of the heat-transferring fluid, as it heats up during use ofthe heat exchange tube, is not confusing, i.e. readily leads to theincorrect assumption that leakage is involved because fluid (due toexpansion) is forced from the capillary leak detection channel.

Further, GB-A-822.705 discloses a heat exchange tube composed of threeparts, such as an outer tube, an inner tube and a helical strip providedbetween those tubes and soldered both to the inner tube and to the outertube. This construction is formed by first slipping the three parts intoone another, with a layer of solder provided between each two parts.Next, the inner tube is expanded, or the outer tube is compressed, toform a mechanical connection between the assembly of outer tube, helicalstrip and inner tube, which mechanical connection is supplemented with asoldered joint by subjecting the assembly, during or after the deformingoperation, to a heat treatment. Over a non-soldered joint, this solderedjoint has the advantage that at the transition between a tube and thestrip, a better, i.e. more complete, joint can be realized. On the otherhand, however, through the use of three parts, the construction is morecomplicated; the parts, when being slipped into each other, can bepositioned relative to each other less accurately due to the presence ofa helical and hence flexible strip, resulting in a leak detectionchannel having an irregular cross section along the length thereof; thedeformation of one of the tubes is to be effected through theinterposition of the helical, flexible, separate strip, resulting in amechanical connection that can be less controlled and defined; and, lastbut not least, there are two transitional areas formed by solder, whichadversely affects the heat transfer because solder, for instance tin,always has a lower coefficient of heat transfer than the materials, forinstance copper, of the parts to be connected.

The object of the invention is to increase the heat transfer to a valueequal or substantially equal to that of a one-piece heat exchange tube,while, moreover, the leak detection channel remains free of fillingmedia and thus fulfills its function directly, accurately and reliably.

In accordance with the invention, this is achieved in a method describedin the preamble, if

-   -   prior to slipping the inner and outer tubes one into the other,        at least the outer surface of the inner tube or the inner        surface of the outer tube is provided with a layer of soldering        material, such as tin;    -   the expansion of the inner tube is effected such that the outer        tube is expanded as well; and    -   the layer of soldering material between inner and outer tube is        caused to melt;        wherein the expansion of the outer tube is effected such that        the molten solder layer is largely forced out between the inner        tube and the outer tube into the at least one leak detection        channel.

Through these features, an optimum contact between inner and outer tubeis created and maintained during use of the heat exchange tube.

By expanding the outer tube by way of the inner tube, the effectachieved is that upon shrinkage of the inner tube due to a decrease intemperature of the heat exchange medium passed therethrough, the outertube, by elastic rebound, always continues to follow the inner tube, sothat the close contact between inner and outer tube is alwaysmaintained.

Accomplishing and maintaining that intimate contact is also effected andsupported by soldering the inner tube and the outer tube together.Protracted tests have shown that, for instance in the case ofcopper/copper contact without a connecting layer, the heat transfer ishighly dependent on the nature of the adjoining copper surfaces, thedegree of contact (air inclusion) and the pressure at the location ofthe abutment. The heat transfer may decrease considerably in the courseof time. It is assumed that the reason for this is the oxidation of theadjoining surface layers, partly as a result of relative movements ofthe surfaces through temperature change during use of the heat exchangetube. By connecting the contacting surfaces with a layer of solder from,for instance, tin, the above effect reducing the heat transfer in thecourse of time has been found not to occur anymore.

Tin has a lower coefficient of heat transfer than copper. It would seem,therefore, that the provision of a layer of tin between two adjoiningcopper surfaces has an adverse effect on the heat transfer. When themethod of the invention is used, however, a heat exchange tube isobtained having a heat transfer which hardly, if at all, differsmeasurably from that of a one-piece copper tube. This surprising effectseems to be the result of the pressure generated between the inner andouter tube by expanding the assembly of those tubes. This pressure issuch that upon melting of the layer of tin, all excess tin is forced outinto the leak detection channel, leaving only a very thin film of tin,which moreover is fused with the adjoining copper surfaces. In thismanner, the copper/copper contact is optimally maintained, with the(connective and filling) tin providing that no mutual detachment throughrelative displacement and hence no oxidation can take place, with theresult that the optimum heat transfer is maintained undiminished, alsoin the course of time during the use of the heat exchange tube.

This effect is partly the result of the use of only two tubes which areslipped one into the other and which are each alone relatively rigid.When, through expansion of the inner tube, the outer tube is expanded,so high a surface pressure is generated on the contact area betweenthose tubes, that during heating of the assembly, the layer of solderpresent between the outer tube and the inner tube is forced outpractically completely, thereby yielding the copper-copper contactalready mentioned. When the assembly consists of three or more parts,during the expansion of the first part, the surface pressure between thesecond and the third part will be less, due to the “loose” second part,because this part, certainly if in addition it is of helical design, canalso deform in axial direction. Partly because the transition can bedetermined less accurately, as observed hereinabove, this may lead tothe layer of solder being insufficiently forced out, if at all, andhence to a reduced heat transfer. Because the heat transfer depends onthe weakest link in the chain, if the heat transfer between the secondand the third part is less, the heat transfer of the entire constructionis lower than the possible heat transfer between the first and thesecond part. Thus, for the present object, a two-piece construction hasthe critical advantages of an accurate reproducibility, an alwaysoptimal heat transfer, which, as stated above, does not measurablydiffer from that of a one-piece heat exchange tube, and a simplemanufacture.

To enable the outer tube to optimally follow the inner tube as thelatter becomes colder, it is preferred, according to a furtherembodiment of the invention, to manufacture the inner tube from a softermaterial than the outer tube. Through this feature, the elastic reboundforce in the harder outer tube will be greater than in the softer innertube, so that the outer tube will in each case be more inclined torebound than the inner tube and the closely abutting contact betweeninner and outer tube is in each case optimally established andmaintained and also the forcing out of the molten solder layer to thedesired degree is always guaranteed still better.

The surface profiling for forming the leak detection channel can beperformed in many ways. In accordance with a further embodiment of theinvention, however, it is preferred that the surface profiling isperformed such that, measured on the respective surface of therespective tube, it occupies at most about 50% of that surface.Extensive test measurements have shown that a double-walled heatexchange tube can then be realized which has optimum leak detectionproperties and a heat transfer which hardly, if at all, differsmeasurably from that of a one-piece tube.

In accordance with the extremely stringent government requirementsapplying in the Netherlands, the leak detection channel must be soarranged that when a through hole of a diameter of 2 mm is drilled inthe heat exchange tube in a critical part thereof, and a water pressureof 50 kPa is applied on both sides of the tube, leakage fluid flowingfrom the leak detection channel must be detected within 300 s. Thisrequirement can be met with a heat exchange tube according to theinvention, and this without loss of heat transfer compared with aone-piece heat exchange tube if, according to a further embodiment, thesurface profiling is provided in the form of a helical groove of a widthof about 2 mm and a pitch of about 4 mm.

Heating the assembly of outer tube and inner tube for melting the layerof solder can be advantageously effected by further heat treatments tobe performed on the heat exchange tube, for instance during heating forsoldering fin-shaped members to at least the outer surface of the outertube or the inner surface of the inner tube, such as a wire spiralhelically wound around the tube.

A layer of solder can be provided on the inner or the outer tube or onboth, independently of the presence and the time of provision of asurface profiling for forming the leak detection channel. In accordancewith the invention, however, it is preferred that when the outer surfaceof the inner tube is coated with a layer of soldering material, asurface profiling in the form of at least one helically extending grooveis subsequently provided therein. If it is preferred to provide asurface profiling in the inner surface of the outer tube, for instancethrough extrusion, then preferably the outer surface of the inner tubeis provided with a layer of soldering material and the inner surface ofthe outer tube is provided with a surface profiling in the form oflongitudinally extending grooves.

Depending on the application involved, it may be advantageous to payparticular attention to the ends of the heat exchange tube to preventsplitting of the two tubes, starting from an end. In that case, it isproposed that a silver weld be provided at each end of the assembly ofinner and outer tube, at the seam between inner and outer tube.

Alternatively or supplemental thereto, it is further possible that at atleast one of the ends of the assembly of inner and outer tube, at leastthe inner surface of the inner tube or the outer surface of the outertube is provided with an insulating coating of lacquer. In this manner,the end in question is shielded from unduly great heat shocks upon asudden change in the temperature of the heat exchange medium beingpassed through, for instance as may occur in central heatinginstallations.

The invention also relates to a heat exchange tube with leak detectioncomprising an assembly consisting of an outer tube and an inner tube inintimately abutting contact therewith, and at least one leak detectionchannel extending in and adjacent to the interface between inner andouter tube, as known, for instance, from DE-A-30 006 65. To realize anoptimum heat transfer in such a heat exchange tube, and also to maintainthis optimum heat transfer in the course of time during use, it isproposed, according to the invention, that at the location of thecontact between inner and outer tube, a film-thin layer from solderingmaterial, such as tin, is present, which is connected to both the innertube and the outer tube by fusion, the inner tube and the outer tubeabutting against each other under a bias, so that the film-thin layercan, as it were, be porous, i.e. locally interrupted.

To render the leak detection channel optimally accessible andoperational without essentially influencing the ends of the heatexchange tube, it is proposed, according to a further embodiment of theinvention, that adjacent an end of the assembly of inner and outer tube,a through opening is provided in the outer tube, which through openingis in communication with the or each leak detection channel provided inthe assembly.

Further protection of the ends against heat shocks is possible if at atleast one of the ends of the assembly of inner and outer tube, at leastthe inner surface of the inner tube or the outer surface of the outertube is provided with an insulating coating of lacquer. If, forincreasing the heat-transferring capacity, fin-shaped members, such as awire spiral helically wound around the tube, are soldered to at leastthe outer surface of the outer tube or the inner surface of the innertube, it may be preferred to omit those fin-shaped members along thelength of the coating of lacquer.

With reference to exemplary embodiments represented in the drawing, themethod, and heat exchange tube to be obtained thereby, according to theinvention, will presently be further discussed, though by way of exampleonly. In the drawing:

FIG. 1 shows in elevation a first assembly of an inner tube and an outertube partly slipped into each other, with a part of the inner tube cutaway;

FIG. 2 shows a cross section taken on the line II—II in FIG. 1;

FIG. 3 shows a cross section corresponding to FIG. 2 of a completed heatexchange tube;

FIG. 4 shows in elevation a second assembly of an inner tube and anouter tube, partly slipped into each other, with a part of the innertube and of the outer tube cut away;

FIG. 5 shows half a cross section taken on the line V—V in FIG. 4; and

FIG. 6 shows a third variant of a heat exchange tube.

FIG. 1 shows an inner tube 1 partly slipped into an outer tube 2. Theinner tube 1 is manufactured from a smooth copper tube whose outersurface has first been provided with a thin layer of tin 3, whereafterfour regularly spaced apart helically extending grooves 4 have beenprovided in the tin-plated surface. The outer tube 2 consists of asmooth copper tube having an inside diameter slightly greater than theoutside diameter of the layer of tin on the inner tube 1.

After the inner tube 1 has been slid completely into the outer tube 2,the assembly thus obtained is subjected to a deforming operation,whereby the inner tube 1, if so desired in more than one step, using adrawing mandril, is expanded and plastically deformed such that thelayer of tin proceeds to abut tightly against the inner surface of theouter tube. To maintain this abutment also upon shrinkage of the innertube 1 due to a decrease in temperature, the expansion of the inner tube1 is continued until the outer tube 2 is expanded as well, in such amanner that an elastic bias is generated in the outer tube 2, whichprovides that the outer tube 2 continues to follow the inner tube 1 uponshrinkage.

After this expanding operation, the assembly is heated to a temperaturesuch that the layer of tin 3 starts to melt. Partly due to the elasticbias in the outer tube 2, the molten tin will start to flow and thereby,on the one hand, fuse with the copper of the two adjoining tube surfacesand, on the other, be pressed from between those two copper surfacesinto the grooves 4. Thus, after the heat treatment, the two coppersurfaces are soldered together in that the tin has filled andsupplemented the two copper surfaces in such a manner that they actuallymerge without interruption. As a result, although the two coppersurfaces are bonded together by the layer of tin, that layer of tin hasat the same time been reduced, through the bias in the outer tube, to anextremely thin, as it were porous, film. This, in turn, has as a resultthat, despite the fact that tin has a lower coefficient of heat transferthan copper, the heat transfer by the assembly of tubes is hardly, if atall, measurably less than that of a comparable solid copper tube, evenif the surface of the groove is as large as that of the residual rib.

Bonding the two surfaces together by means of the tin has as aconsequence, inter alia, that during heat movements, shrinkage orexpansion, no displacements between the two surfaces occur. This, andthe fact that by filling up with tin any small irregularities betweenthe two surfaces and forcing the excess tin into the grooves, anyinclusions of air are removed, prevent oxide formation on the coppercontact surfaces, and in particular slowly inwardly progressingoxidation of the copper surfaces at either of the ends (splittingthrough notch effect). Since oxide formation has a highly adverse effecton the heat transfer, what is thus accomplished as well is that theproper heat transfer of the composite heat exchange tube according tothe invention, which, as stated, is comparable to that of a solid coppertube, is maintained also in the course of time during use.

FIG. 2 shows the assembly of inner tube 1 and outer tube 2 before theexpanding operation; FIG. 3 shows this assembly after termination of theheat treatment, i.e. in the completed condition. This is expressed inFIG. 3 in that it no longer shows the layer of tin, reduced to anextremely thin, if not porous, film, but indicates the excess tin,forced into the grooves 4, as solidified drops 3′. In FIG. 3 it isfurther indicated that the tubes have been expanded with respect to thesituation in FIG. 2, i.e., all diameters of the tubes have increased,while further the outside diameter of the inner tube 1 has become equalto the inside diameter of the outer tube 2.

It is noted that the various dimensions are not shown to scale, which istrue in particular of the layer of tin 3. Hereinbelow, it is indicated,exclusively by way of example, how a composite heat exchange tube havingan outside diameter of 28.3 mm and an inside diameter of 23 mm can beobtained.

The starting point is an inner tube of half-hard copper having anoutside diameter of 25 mm and an inside diameter of 22 mm and an outertube of hard copper having an outside diameter of 28 mm and an insidediameter of 25.6 mm. After the tubes have been inserted one into theother and expanded in two steps, a composite heat exchange tubefunctioning as a one-piece heat exchange tube is obtained, having anoutside diameter of 28.3 mm and an inside diameter of 23 mm, thetransition (tin film) between the inner and outer tubes being located ata diameter of 26 mm. The total wall thickness involved has decreasedfrom 2.7 mm to 2.65 mm. This is a result of the cold deformation(expansion) whereby the composite tube becomes slightly longer. Themeasurements mentioned have been selected after it had been established,through tests, that at such a degree of expansion, the elastic reboundof the outer tube is sufficient for the outer tube to follow a suddenshrinkage of the inner tube due to a temperature shock of 100° C. to 10°C. without itself needing to decrease in temperature.

The choice of material (half-hard copper for the inner tube and hardcopper for the outer tube) promotes the desired elastic rebound effect,because softer material rebounds less than harder material. In theexemplary embodiment according to FIGS. 4 and 5, a copper inner tube 11,provided with a layer of tin 13, has been inserted into an outer tube 12whose inner surface comprises fifteen grooves 14, for instance obtainedby extrusion, extending in longitudinal direction of the tube. Thesituation in which the two tubes are shown is identical to that of FIG.1, i.e., after the tubes have been completely slipped one into theother, expansion will take place to the degree described in theforegoing, after which, through a heat treatment, the layer of tin 13 iscaused to melt, whereby the excess tin at the location of thelongitudinal ridges on the outer surface of the inner tube 11, asdescribed hereinabove, is forced out, to form a residual, filling andconnective film of tin, into the longitudinal grooves 14 forming a leakdetection channel, thus yielding a composite heat exchange tube withheat detection functioning as a one-piece heat exchange tube and havinga configuration comparable to that according to FIG. 3.

FIG. 6 shows a heat exchange tube comprising an inner tube 21 and anouter tube 22, tightly abutting against each other and connected througha film of tin, all in a manner as discussed hereinabove. At thetransition between the two tubes 21, 22, a single helical groove 24 hasbeen provided in the outer surface of the inner tube 21, which grooveforms a leak detection channel. Such a leak detection channel isprescribed in situations where the heat-releasing medium must never comeinto contact with the heat-absorbing medium. If a crack is formed in theinner or the outer tube, medium leaking therethrough will end up in theleak detection channel. To be able to establish the presence of fluid inthe leak detection channel, this should be perceptible. For this reason,an opening 25 has been provided in the outer tube 22, which opening isin open contact with the leak detection channel. The opening 25 may bein communication with a leak detection means detecting the medium whichhas leaked or a change in pressure.

It has already been mentioned that the splitting of the composite heatexchange tube is extremely disadvantageous to the heat transfer, and howsuch splitting is prevented in the present heat exchange tubes. Afurther safeguard in this regard can be provided through the provisionof a silver weld 26 (see FIG. 6) at the transition between the innertube 21 and the outer tube 22, at at least one of the ends of thecomposite heat exchange tube. In addition to or instead of thisreinforcement, it may also be provided that the end in question is lessexposed to heat shocks through the provision of an insulating coating oflacquer 27 (see FIG. 6).

To increase the heat transfer, fins or ribs may be provided on the outersurface of the outer tube 22 or the inner surface of the inner tube 21.Such fins or ribs can be formed through extrusion. Another possibilityis the provision of a helically wound wire 28 (for instance having atrapezoidal winding profile; see FIG. 6) which is subsequently woundhelically around the outer tube 22. Connecting such wire to a tube iseffected by soldering. This heat treatment may simultaneously serve tomelt the layer of tin between the inner tube and outer tube to obtain acomposite heat exchange tube functioning as in one piece, as describedin the foregoing.

In the embodiment according to FIG. 6, the inner tube is likewiseprovided with fin-shaped members, again in the form of a helically woundwire 29 helically wound around and fixed on a support tube 30concentrically inserted into the inner tube 21. If desired, the innersurface of the inner tube 21 may be tin-plated, so that during the heattreatment referred to above, the ends of the wound wire 29 remote fromthe support tube 30 are secured to the inner surface of the inner tube21.

It is readily understood that within the framework of the invention aslaid down in the appended claims, many more modifications and variantsare possible. Thus, for forming a leak detection channel in theabove-discussed exemplary embodiments, grooves are provided in the innersurface of the outer tube or the outer surface of the inner tube. Ofcourse, grooves may also be provided in both surfaces, or the variousgrooves may be interconnected by further grooves, yielding a more orless knurled surface. Further, copper and tin are mentioned asapplicable materials; however, this does not exclude the use of othermaterials. Further, under certain circumstances, it is possible to omitthe heat treatment for melting and partly forcing out the layer of tin,for instance when the expansion is accompanied by a heat developmentsuch that the soldering material already melts during expansion.

1. A method for manufacturing a double-walled heat exchange tube with aleak detection channel; the method comprising the steps of: providinginner and outer tubes, with the outer tube having an inner surface andthe inner tube having an outer surface, the inner tube beingmanufactured of a softer material than the material of the outer tube;providing a surface profiling on at least one of the inner surface ofthe outer tube and the outer surface of the inner tube; selectivelyproviding at least one of the inner surface and said outer surface witha layer of soldering material; expanding said inner tube such that theouter surface of the inner tube is in intimate contact with the innersurface of the outer tube and the surface profiling forms at least oneleak detection channel between the two tubes; expanding the inner tubesuch that the outer tube is expanded as well; causing the layer ofsoldering material between the inner tube and the outer tube to bemelted; wherein the expansion of the outer tube is effected such thatthe molten solder is forced out between the inner tube and the outertube into the leak detection channel.
 2. The method in accordance withclaim 1 wherein the surface profiling is carried out of such that itoccupies at most 50% of a surface.
 3. The method in accordance withclaim 2 wherein the surface profiling in the form of a helical groovehaving a width of about 2 mm and a pitch of about 4 mm.
 4. The method inaccordance with claim 1 wherein the heating takes place by soldering awire spiral would helically on a surface of at least one of inner tubeand outer tube.
 5. The method in accordance with claim 1, wherein theouter surface of the inner tube is coated with a layer of solderingmaterial and a surface profiling comprising at least on helicallyextending groove is provided.
 6. The method in accordance with claim 1,wherein the outer surface of the inner tube is provided with a layer ofsoldering material and the inner surface of the outer tube is providedwith a surface profiling in the form of longitudinally extendinggrooves.
 7. The method in accordance with claim 1, wherein a silver weldis provided at a seam between the inner tube and the outer tube.
 8. Themethod in accordance with claim 1 wherein at at least one of the ends ofthe assembly of the inner tube and the outer tube at least one of theinner surface of the inner tube and the outer surface of the outer tubeis provided with an insulating coating of lacquer.
 9. A heat exchangetube for use in a heat exchanger employing a liquid and comprising: anassembly of an outer tube and an inner tube disposed internally to saidouter tube and retained in an abutting position under a bias pressure,to form an inner face between said inner tube and said outer tube; aleak detection channel extending adjacent said inner face; a throughopening extending through said outer tube at a position adjacent an endof said assembly of said inner tube and outer tube, said through openingbeing in communication with the leak detection channel; and a film-thinlayer formed of a soldering material disposed in contact with both theinner tube and the outer tube and wherein the inner tube and the outertube are retained in abutting contact under the bias pressure.
 10. Theheat exchange tube in accordance with claim 9 wherein at least one ofthe ends of said assembly is provided with an insulating coating oflacquer on at least one of the inner surface of the inner tube and theouter surface of the outer tube.
 11. The heat exchange tube inaccordance with claim 9, wherein one of the outer surface of the outertube and the inner surface of the inner tube is provided with fin-shapedmembers.
 12. The heat exchange tube in accordance with claim 9, whereinfin-shaped member are soldered on at least an outer surface of saidouter tube and wherein the fin-shaped members comprise a wire spiralhelically wound around the outer tube and soldered to the outer tube.13. The heat exchange tube in accordance with claim 9, wherein a surfaceprofiling measured on a surface of one of said tubes occupies at most50% of said surface.
 14. The method according to claim 9 wherein saidsurface profiling comprises a helical groove having a width of 2 mm anda pitch of 4 mm.